Hydration Mechanism of Portland Cement Prepared ...

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ScienceDirect Procedia Environmental Sciences 31 (2016) 598 – 604

The Tenth International Conference on Waste Management and Technology (ICWMT)

Hydration mechanism of Portland cement prepared from stonecoal vanadium slag Pengyu Zhanga,b, Leming Oua,b, Kun Liua,b,* b

a School of Minerals Processing & Bioengineering, Central South University, Changsha 410083, China Key Laboratory for Mineral Materials and Application of Hunan Province, Central South University, Changsha 410083, China

Abstract This paper presents an objective study on the utilization of stone coal vanadium slag in preparing cement clinker. The hydrates and hydration mechanism of this cement were analyzed and studied by means of the hydration heat analysis, X-ray diffraction (XRD) and the differential thermal gravity (DTG) analysis. The results of experiments show that the hydration mechanism is similar to ordinary Portland cement. The hydration process can be divided into five stages: (I) initial period; (II) induction period; (III) acceleration period; (IV) deceleration period; (V) final period. And the hydrates are basically the same as Portland cement, mainly containing the calcium silicate hydrates (C-S-H), ettringite (AFt), portlandite (CH). It is proved that stone coal vanadium slag can be used as siliceous materials to prepare cement clinker. Furthermore, the addition of fine materials such as the waste and fly ash can accelerate cement hydration, which is the result of giving rise to water-to-cementitious ratio. On the other hand, the fine materials may provide the crystal nucleus for hydrates such as portlandite. Using the waste and fly ash to replace part of clinker can prepare series of cement, whose compositions and physical properties are fully complied with the requirements of national standard, and bring huge ecological and economic benefits. © 2016 2015The TheAuthors. Authors. Published by Elsevier B.V.is an open access article under the CC BY-NC-ND license © Published by Elsevier B.V This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Tsinghua University/ Basel Convention Regional Centre for Asia and the Pacific Keywords:solid waste; stone coal vanadium slag; hydration mechanism; cement clinker

* Corresponding author. Tel.: +86-731-88877204; fax: +86-731-88877204. E-mail address: [email protected]

1878-0296 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Tsinghua University/ Basel Convention Regional Centre for Asia and the Pacific doi:10.1016/j.proenv.2016.02.102

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Pengyu Zhang et al. / Procedia Environmental Sciences 31 (2016) 598 – 604

1. Introduction With the development of China’s mining industry, more and more mineral waste residue is discharged year by year. These solid wastes cause varieties of resources and environmental problem on people’s living area. While at the same time, the demands of cement are increasing with the remarkable development of the construction industry. Thus, in recent years, utilization of proper solid waste to prepare cement has become an efficient way to deal with the solid mineral wastes problems1-7. Stone coal is a type of inferior anthracite, in which the grade of vanadium is less than 1%. In China, it commonly extracts vanadium resources from stone coal by hydrometallurgical extraction process. In the process, a great deal of solid waste (named “vanadium slag”) is inevitably produced. The vanadium slag mainly composites of quartz and silicate minerals. Due to this characteristic, it can be used as siliceous and correction materials to prepare cement clinker. Relevant researches about how to use vanadium slag to prepare cement clinker had been carried out and the qualified cement clinker production had been produced. However, to date, there has been relatively little research conducted on the mechanisms of the production. Especially, the hydration mechanism and property of the cement clinker production are unknown. These mechanisms are very important for the industrial design and practical production. Thus, the purpose of the current research is to characterize the hydration mechanisms of different classes cement prepared from vanadium slag. 2. Experiment 2.1. Raw materials The cement clinker used in the current research is prepared by self in laboratory. It consists of following steps: (1) stone coal vanadium slag (3-16 wt%) blended with nitric phosphate slag (76-79 wt%) and other various aluminum and iron materials (7-20 wt%); (2) heated at 1350-1450 ć and then cooled to room temperature; (3) ground to granularity less than 0.074 mm. The waste used is from mining tailings and fly ash comes from thermal power plants. The mineralogical composition of cement clinker is listed in Table 1, and the chemical composition of materials is listed in Table 2. Series preliminary experiments about material proportion have been conducted before the research. Among these experiment results, the material ratios of samples complied with the requirements (national standard of China) are picked out. The samples of B, C and D are up to the standard of 52.5#, 42.5# and 32.5# cement. The proportions and physical properties of samples are listed in Table 3. Table 1 Mineralogical composition of cement clinker C3S (%)

C2S (%)

C3A (%)

C4AF (%)

56.62

19.08

7.98

10.23

Table 2 Chemical composition of materials SiO2 (%)

Al2O3 (%)

Fe2O3 (%)

CaO (%)

MgO (%)

SO3 (%)

LOSS (%)

Σ (%)

Clinker

21.56

5.16

3.37

65.01

2.12

0.40

0.27

97.89

Waste

62.31

10.33

6.71

6.83

1.29

0

8.67

96.14

Fly ash

53.17

27.68

6.08

2.88

1.14

0.31

7.52

98.78

2.2. Methods Cement paste samples were prepared at the w/c (water/cement) of 0.4. The cement paste was placed in a cubic paste mould (20×20×20 mm) and cured for 24h in a certain atmosphere (temperature of 20±2 ć and relative humidity of above 90 %). Then, the formed cement cubes were sequentially maintained in water with temperature of 20±1 ć for a certain time. After the curing, the cement samples were crushed into small pieces and immersed into

600


ethanol for more than 20h to discontinue the hydration (the volume ratio of ethanol to cracked pieces was about 10:1). Cement paste samples were there taken out of ethanol and oven dried at 60 ć for at least 6h. The samples were pulverized using an agate pestle and mortar. The particle size of the final samples should less than 80 µm. Hydration heat was measured by a SETARAM BT2.15I Calvet-type microcalorimeter with temperature of 20 ć and water cement ratio (by mass) of 2:1. The XRD analysis was performed on a fully automatic X-ray diffraction apparatus of Rigaku D/max 2550 VB+ with Cu Kα radiation, an acceleration voltage of 40 kV and an emission current of 100 mA at a scanning rate of 2° per minute. The DTG analysis was carried out on a NETZSCH STA449C thermal analyzer with temperature from 50 to 1100 ć, heating rate of 10 ć/min and air atmosphere. Table 3 Proportions and physical properties of samples Code

Clinker (%)

Waste (%)

Gypsum (%)

Fly ash (%)

Strength of 3d (MPa)

Strength of 28d (MPa)

Flexural

Compressive

Flexural

Compressive

A

100.0

0

0

0

6.9

33.8

9.9

60.9

B

95.0

0

5.0

0

6.5

32.1

9.5

58.4

C

78.5

8.5

5.0

8.0

5.1

23.5

8.9

47.1

D

60.5

25.0

4.5

Code

10.0

Water demand for normal consistency (%)

Average particle (μm)

A

23.40

B

27.90

C D

4.4

18.6

7.0

Specific surface area (m2/g)

LOSS (%)

Soundness

2.2

359

0.27

2.4

347

1.41

27.80

3.6

345

26.00

5.8

399

size

36.2 Setting time (min) Initial

Final

Qualified

116

160

Qualified

239

292

4.62

Qualified

207

263

11.90

Qualified

217

275

3. Results and discussion 3.1. Hydration heat Fig. 1 shows the hydration heat release rate curves of the samples at 20 ć. The hydration process can be divided into five stages8: (I) initial period; (II) induction period; (III) acceleration period; (IV) deceleration period; (V) final period. Moreover, there are commonly two exothermic peaks in the cement hydration process. The first exothermic peak is in initial period. It is mainly caused by ion dissolution and reaction between C3A and calcium sulfate, usually resulting in formation of ettringite. The second exothermic peak is in acceleration period. It mainly caused by hydration of C3S, forming calcium silicate hydrates (C-S-H) and calcium hydroxide. It can be seen in the Fig. 1 that the first peak of sample B is higher than that of sample C and sample D. It is because a relatively lower content of admixture and higher content of C3A in sample B. Sample A is all clinker, which has little ettringite and hydration heat. The SO42- in gypsum can significantly reduce the solubility of aluminate, which lead to prolonged hydration and delay the presents of induction period of sample B, sample C and sample D9,10. Moreover, from Fig. 1, the hydration heat rate of sample D and sample C is significantly higher than that of sample A. Besides, the acceleration time is relatively shorter. Obviously, the partial replacement of the cement by fine materials (mainly fly ash and high-silicon waste stone) accelerates the cement hydration reaction. These fine minerals are served as crystal nucleus to ensure the growth of hydrates (such as portlandite) 11,12. The additional of admixture result in a faster separation out of portlandite and decreasing the concentration of Ca 2+. Thus, in this case, the cement hydration is accelerated and the hydration of the second exothermic peak is increased. Comparing to sample C, sample D adds with more mineral admixture, which makes a faster crystallization velocity and higher second exothermic peak. Moreover, in the acceleration period, the hydration heat release rate of sample C is lower than sample B. It attributes to the slow pozzolanic reaction of fly ash. By using the same amount of fly ash to replace clinker, it can significantly reduce the temperature peak13-15.


Fig. 1. Heat evolution rate curves of samples at 20 ć Iüinitial period; IIü induction period; IIIüacceleration period; IVüdeceleration period; Vüfinal period

The hydration of the fly ash requires the alkaline excitation of the cement hydration product 16,17. So, a certain period of time is necessary for destroy the vitreous surface and the integument formed by the cement hydration product. In the later stage, the activity of fly ash increases and the pozzolanic effect takes action, which makes a higher hydration release rate. The completely hydrated cement clinker (mainly C2S) in sample B begins to release hydration heat slowly. Moreover, because of the largest amount of admixture dosage (25 %, mass fraction), the cement clinker content in sample reduces. It results in lower exothermic rate of sample D than sample B after 42 hours later. 3.2. XRD Samples hydrated for 3d and 28d are analyzed by X-ray diffraction analyzer, as presented in Fig. 2. The main hydration products are basically the same. They are the ettringite (AFt), portlandite (CH), traces of CaCO3 (from carbonation of portlandite), unhydrated clinker and unreacted admixtures. Compared to sample A and sample B, sample C added with more admixtures (16.5 wt%), resulting in a more diffuse peaks as illustrated in Fig. 2. With the further increasing amount of admixtures, it makes the diffuse peaks of sample D further increased. Contrast with Fig. 2(a) and Fig. 2(b), it can be seen that the clinker in samples is reducing along with the increasing of hydration time. At the same time, the hydration products present an increasing trend under this condition. The content of clinker minerals (C3S and C2S) and hydration products of portlandite under different hydration time were semi-quantitative analyzed from the X-ray diffraction data. The results are shown in Fig. 3 and Fig. 4. It can be seen in Fig. 3 that the content of C3S and C2S sharply declines during 24h. When the hydration time exceeded 24h, this change becomes not evident. The main reason is amount of hydration products quickly generated at the beginning18,19. These generated hydration products would cover on the unhydrated particle surface and decelerate the hydration rate. The sequence unhydrated C3S and C2S content is: A, B, C, D. The sequence of clinker minerals reducing rate is: D, C, B, A. The reason for that lies in two aspects. On one hand, the addition of admixtures lowers the total content of clinker minerals, which results in a less content of C3S and C2S in the samples. On the other hand, the admixtures accelerate the hydration rate due to its nucleation, which leads to a more obvious clinker reduction rate of sample C and sample D. After 28d hydration, unhydrated clinker was still existed. The main reason is the slow hydration rate of C2S.

601

602


Fig. 2. X-ray diffraction (XRD) patterns of samples hydrated for 3 d (a) and 28 d (b)

Fig. 3. Variations of C3S and C2S in different samples vs hydration time

Fig. 4. Variations of Ca(OH)2 in different samples vs hydration time

From Fig. 4, it can be seen that the content sequence of portlandite is: A, B, C, D. It is because the addition of admixtures lowers the clinker content, resulting in a less formation of hydration products. The SiO2 in admixtures gradually react with portlandite and overbasic calcium silicate hydrates to generate low-alkaline calcium silicate hydrates20. Moreover, Ca(OH)2 would also react with Al2O3 in the admixtures and generate calcium aluminate hydrates, which increasing the consumption of Ca(OH) 2 (as presented in Fig. 4). The Ca(OH)2 content decreased after the samples hydrated 3d, as the result that the hydration of clinker (C3A, C4AF, C3S) has reached the final period within 72h. And at the same time, because of the slow hydration rate of C2S, the generation of Ca(OH)2 would relatively reduce. 3.3. DTG The thermal decomposition characters of the sample were examined by the differential thermal gravity (DTG) analysis. The results are shown in Fig. 5. From the diagram, it can be calculated that the hydration products are


basically the same as Portland cement, which is agree well with XRD analysis results, These hydration products are mainly composited of calcium silicate hydrates (CSH), ettringite (AFt), portlandite (CH) and CaCO 3.

Fig. 5. Differential thermal gravity (DTG) spectra of samples hydrated for 3 d (a) and 28 d (b)

Moreover, from Fig. 5, the first endothermic valley is at the point of 110 ć and the corresponding temperature range is 30 to 200 ć. Under this temperature region, it is mostly the peak of aqueous phase dehydration, including inter-calcium silicate hydrates (CSH), ettringite (AFt) and single sulfur-type monosulfate (Afm)21,22. The dehydration temperature is different due to the combinative state of water in each product is different. Samples hydrated for 28 days have deeper valley than that of 3 days. It is mostly caused by the continuous generation of C-SH and other products. Because part of clinker replaced by admixtures the hydration products is relatively reduces. It makes the valley of sample C and sample D are lower than A’s. This phenomenon on sample D contained with 25 wt% waste material is most prominent. After hydrated for 28 days, the pozzolanic activity of admixtures appears. It makes the first valley depth of sample C and sample B nearly the same. The second endothermic valley is in the vicinity of 470 ć and the corresponding temperature region is 430 to 500 ć. It is mainly because that the Ca(OH)2 in cement decomposes and draws lots of heat at this point23. According to the Fig. 5, the depth of the second valley is increasing along with the time, which can be attributed to the constant generation of portlandite in cement24. In addition, from Fig. 5, sample C and sample D have shallower valley. There are two reasons for that. Firstly, part of clinker replaced by admixtures results in a relative reduction of hydration products. Secondly, due to the pozzolanic activity of fly ash, it consumes portlandite for a secondary hydration. The third endothermic valley is around 800 ć and the corresponding temperature range is 600 to 900 ć. This endothermic process is mainly caused by the thermal decomposition of CaCO3 and the dehydration of hydrated silicate structure water25. 4. Conclusions The hydration mechanism of the cement clinker (prepared from stone coal vanadium slag as siliceous materials) is similar to ordinary Portland cement. The hydration products, mainly containing the calcium silicate hydrates (CS-H), ettringite (AFt) and portlandite (CH), are basically the same with Portland cement. During the hydration process, the fine materials (mainly the waste and fly ash) could provide crystal nucleus for hydrates such as portlandite, and accelerate cement hydration. The fly ash in cement paste has pozzolanic activity under the alkaline excitation. As well, the performance of cement prepared from stone coal vanadium slag conforms to national standards of China. As a result, the stone coal vanadium slag can be used as raw materials to produce qualified cement products, which brings ecological and economic benefits greatly.

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